Grease and line a 30cm 20cm baking tray. 
Pre-heat the oven to 180C/350F/gas 4. 
Put the flour, sugar and butter into a processor.
Whizz everything together; first it will turn to crumbs, pulse until it forms into a ball.

Press into the prepared tray and bake for 20 minutes until firm and pale golden

Put the condensed milk, sugar and butter into a bowl and microwave on High for eight minutes, stirring every two minutes until the mixture IS thick and fudge-like.

Pour onto the shortbread and smooth over. For the topping, melt the chocolate in a bowl in the microwave on Medium (5OOW) for two minutes or melt in a saucepan over a bowl of water, stir and pour over the caramel.
Leave to set, then cut.